The present invention relates to electric motors and more particularly to power circuits in electric motors and still more particularly to a direct current electric motor having an assembly for suppressing radio frequency interference.
Electric motors include a stator and a rotor which rotates about a central axis in response to opposing magnetic fields between the stator and the rotor. The opposing magnetic fields are typically induced by electric currents which flow in wire windings in the stator and rotor. Electric motors thereby convert electrical energy to mechanical energy in a rotating shaft which is attached to the rotor.
The electrical energy is supplied to a motor by electrical conductors which are connected to an external supply of electricity such as a battery or alternating current power lines. It is advantageous to provide a stable electrical power source to certain electric motors. For example, direct current (DC) motors require a stable direct current input having substantially no alternating current component. Electrical power circuits typically provide electricity including noise such as voltage surges and transient components. Electrical noise may be caused by any number of phenomena including, for example, power drains and surges and radio frequency disturbances which may be induced by alternating electromagnetic fields caused by near by electrical equipment.
Electrical power that is transmitted even a short distance through conductors acquires induced noise that was radiated through space. It is therefore impossible to provide a perfectly clean power signal over any substantial distance. Devices such as some electric motors which require clean power employ filter circuits proximate to their power input connections to remove most unwanted interference components in the electrical power signal.
Radio frequency interference (RFI) is typically filtered from power signals by filters comprising inductors and capacitors. It has been common practice in the motor industry to use common axial inductors in conjunction with common capacitors in the motor input circuit connected together as filter circuits which are well known in the electrical arts.
The electrical parameters or values of inductors and capacitors are related to their physical size. The effectiveness of a filter is related to the circuit parameters and therefore related to the physical size of the inductors and capacitors.
Typically, RFI suppression components are mounted on the outside of the motor or alternatively a motor package size is increased to accommodate the additional components. The amount of RFI suppression achieved by known designs is often limited by size constraints which are imposed upon the inductor and capacitor components.
Typical RFI suppression circuits are constructed by assembling the inductor and capacitor components to a motor assembly and electrically connecting them together to form the proper filter circuit. There is a need in the art for an RFI suppression package which reduces the size and complexity of the RFI suppression components and incorporates them into a package to speed production and maximize RFI suppression.